During use, many electronic components generate heat. For proper functioning of the electronic component, it is necessary to remove the heat from the component. In particular, advanced integrated circuit devices like CPU in personal computers produce increased amounts of heat due to the acceleration of processing speed. Thus thermal management is of significance.
For heat removal, a number of methods and a variety of heat conductive materials have been proposed. The heat conductive materials generally take two forms, a sheet form which is easy to handle and a paste form commonly known as heat dissipating grease. Since the thermal resistance of a heat dissipating material is in proportion to its thickness, the heat dissipating grease which can be readily thinned by compression has better heat dissipating properties. For electronic components with more heat generation, it is proposed to dispose a heat conductive grease or sheet between the electronic component and a heat sink for efficiently releasing the heat from the electronic component (see JP-A 56-28264 or Aakalu et al. U.S. Pat. No. 4,265,775 and JP-A 61-157587).
However, since the heat release of electronic devices such as LSI is increasing, conventional heat dissipating grease fails to provide satisfactory heat dissipation. The function of heat dissipating grease may be enhanced by heavier loading of heat conductive filler. The heavier loading of heat conductive filler, however, increases the viscosity of grease. The necessary ease of application poses a certain limit to the filler loading. Another approach is to reduce the coating thickness of heat dissipating grease because its thermal resistance is proportional to its thickness. To this end, a heat conductive filler having a smaller average particle size is generally used although no satisfactory heat dissipating effect is yet available. This is because the heat conductive filler, despite a smaller average particle size, often contains incidental coarse particles which prevent the heat dissipating grease from being coated to the desired thickness.
Known heat conductive materials include a heat dissipating grease based on silicone fluid which is loaded with zinc oxide or alumina powder (see JP-B 52-33272 and JP-B 59-52195).
The use of aluminum nitride powder for improving heat transfer is also known. U.S. Pat. No. 4,265,775 discloses a thixotropic thermally conductive material comprising a liquid organo-silicone carrier, silica fibers, and one or more thermal filler powders selected from among dendritic zinc oxide, lamellar aluminum nitride, and lamellar boron nitride. JP-A 2-153995 discloses a silicone grease composition comprising a specific organopolysiloxane and a spherical hexagonal aluminum nitride powder having a certain particle size range. JP-A 3-14873 discloses a heat conductive silicone grease using a combination of an aluminum nitride powder having a smaller particle size with an aluminum nitride powder having a larger particle size. JP-A 10-110179 discloses a heat conductive silicone grease using a combination of an aluminum nitride powder with a zinc oxide powder. JP-A 2000-63872 discloses a heat conductive grease composition using an aluminum nitride powder which has been surface treated with organosilane.
Aluminum nitride has a thermal conductivity of 70 to 270 W/mK while one typical material having a higher thermal conductivity is diamond having a thermal conductivity of 900 to 2,000 W/mK. JP-A 2002-30217 discloses a heat conductive silicone composition comprising a silicone resin, diamond, zinc oxide, and a dispersant.
Metals have a high thermal conductivity and may be used where the electrical insulation of electronic components is unnecessary. JP-A 2000-63873 discloses a heat conductive grease composition comprising a base fluid such as silicone fluid in admixture with a metallic aluminum powder.
However, all these heat conductive materials and heat conductive greases fail to comply with the increased heat release of advanced IC devices such as CPU.
A material based on a silicone fluid filled with a heat conductive filler has a thermal conductivity which is little dependent on the thermal conductivity of the filler if the volume fraction of the filler is equal to or less than 0.6, as understood from the theoretical equation of Maxwell or Bruggeman. The thermal conductivity of the filler becomes significant only when the volume fraction of the filler is in excess of 0.6. This suggests that an increase in the thermal conductivity of heat conductive grease is first dependent on how to heavily load the grease with a heat conductive filler and if heavy loading is possible, how to select a filler having a higher thermal conductivity. However, the heavy loading interferes with the flow of heat conductive grease and detrimentally affects the efficiency of application like coating and dispensing, making the grease unacceptable on practical use.
The other approach is to reduce the coating thickness of heat dissipating grease because its thermal resistance is proportional to its thickness. To this end, a heat conductive filler having a smaller average particle size, with coarse particles cut off, is used although no satisfactory heat dissipating effect is yet available. This is because the heat conductive filler having a smaller average particle size has a larger surface area, which allows for progress of oxidation if the filler is of metal. As a result, the thermal conductivity of filler is reduced, and heavy loading becomes difficult. It is then difficult to impart a high thermal conductivity.
JP-A 2004-091743 discloses a heat conductive grease comprising 15 to 35% by weight of an organopolysiloxane, 35 to 55% by weight of a spherical alumina powder having an average particle size of 0.2 μm to less than 1.0 μm, and 30 to 50% by weight of an aluminum nitride powder having an average particle size of 1 to 3 μm and a maximum particle size of 2 to 10 μm.